Sarcomonad ribosomal RNA sequences, rhizopod phylogeny, and the origin of euglyphid amoebae

We have sequenced the 18S rRNA genes of six sarcomonad flagellates from the orders Cercomonadida (Heteromita globosa and four Cercomonas species) and Thaumatomonadida (Thaumatomonas sp.). Phylogenetic analysis by maximum likelihood; parsimony and two distance methods shows that these two orders are related, supporting their classification in a revised class Sarcomonadea. Molecular divergence within Cercomonas is huge: as great as among all fungi or land plants. Heteromita, Thaumatomonas and euglyphid testate amoebae all apparently evolved from Cercomonas-like ancestors. Euglyphids and thaumatomonads, which both bear silicious scales, are more closely interrelated than are some species of Cercomonas. The Glade comprising sarcomonads and Testaceafilosia is a sister to the amoeboflagellate chlorarachnean algae. Our trees robustly show that sarcomonads, euglyphids, and Chlorarachnea are together specifically related to Plasmodiophora, previously classified in the opalozoan class Phytomyxea: these taxa are now all included in a revised phylum Rhizopoda. Phylogenetic analysis of rRNA sequences representing all major lineages of neokaryotes (i.e. eukaryotes branching above Euglenozoa on molecular trees) shows that Rhizopoda are one of thirteen major neokaryote Glades and are phylogenetically distinct from lobose amoebae (subphylum Lobosa of the phylum Amoebozoa). Rhizopoda may be one of the earliest diverging megakaryote Glades.